(a) Field of the Invention
The present invention relates to a resist processing system and, more particularly, to an air filter of a resist processing system for forming a resist film pattern for use in etching or ion implantation process.
(b) Description of the Related Art
Higher integration and operational speed of semiconductor devices are resulting from advance of higher resolution of optical lithography. Conventionally, g-line lithography in 0.436 nm wavelength is used for semiconductor devices in design rule more than 0.5 .mu.m, and i-line lithography in 0.365 .mu.m wavelength is used for semiconductor devices in design rules between 0.5.about.0.35 .mu.m. KrF excimar laser lithography in 0.248 .mu.m wavelength is used for semiconductor devices in their design rules in 0.30.about.0.20 .mu.m.
A positive type resist such as Novolak resin and Naphthoquinonediazide (NQD) compound are widely used for i-line and g-line lithography. The positive type resist is, however, not applicable for KrF excimar laser lithography because the light absorption rate of Novolak resin is too large against KrF excimar laser having 248 nm wavelength. Resist sidewall will not be formed in a vertical profile and a low illumination of KrF excimar laser requires a resist having a higher sensitivity than conventional resists.
In view of the above, chemically amplified resists are developed for KrF excimar laser lithography. The chemically amplified resist comprises a base resin, an acid generator for generating acid during exposure thereof, and a compound having a reactive radical exhibiting variable dissolution in developer solvent depending on acidity. As a base resin, a phenol-based resin is mainly used which has a high penetration rate for KrF excimar laser having 248 nm wavelength. Acid catalyzer in the chemically amplified resist accelerates chemical reactions for higher sensitivity suitable for a smaller design rule than conventional resists.
It is known that performance of the chemically amplified resists depends on resist processing environment for forming a pattern. For example, the top portion of resist is often formed in a poor sidewall profile such as T-tops when a positive type chemically amplified resist is used because function of the acid catalyzer is impaired by airborne acid contaminants to retard dissolution of the top of the resist.
In an extreme case, adjacent patterns are often bridged together resulting in difficulty in etching the underlying layer.
Patent Publication No. JP-A-06 (1994)-208947 describes that an air filter comprising acid substances reduces by filtering alkali contaminants in a resist processing environment for using chemically amplified resist. Although this method is effective for removing alkali contaminants, replacement of the air filter is required, which lowers productivity of the semiconductor fabrication and causes cost increase. In addition, timing of the air filter replacement is difficult to determine unless processing environment is monitored by an expensive monitor.
Patent Publication No. JP-A-07 (1995)-142312 describes a neutralization reaction chamber wherein airborne alkali contaminants are filtered by either acid gas or acid solution. Although this method may also be effective for neutralizing airborne alkali contaminants, it is difficult to obtain an accurate neutral condition because acidity in the processing environment fluctuates depending on airborne alkali levels.
For example, if the concentration of the acid gas is adjusted to a high level of airborne alkali, the acidity in the processing environment becomes higher when airborne alkalinity is lower. Under this condition, the resist film will be thin in the case of a positive type resist, T-tops sidewall profile will be formed in the case of a negative type resist. A similar problem will arise in a method using the acid solution.
A known air purification equipment comprises an air filter having a combination of a wet scrubber and a dry filter. The wet scrubber uses a water sprinkler for sprinkling water from the top thereof for air purification. This method is targeted at the removal of sodium (Na) and may be used for removal of alkali contaminants. The increase of humidity within the processing environment, however, causes a problem. After alkali contaminants are neutralized, a drying process is required to control the humidity of the processing environment by using a dryer, which causes to make the air filter apparatus larger and equipment cost higher.
As described above, conventional techniques do not provide a long life, simple means for removing airborne alkali contaminants.